1. Field of the Invention
This invention relates to aircraft and more particularly to fail safe operation of electronically controlled positioning actuator systems.
2. Description of the Prior Art
In the aforementioned copending application, there is disclosed a helicopter having an elevating stabilizer, or stabilator, which is positioned, in response to functions of airspeed, collective pitch, lateral acceleration, and pitch rate, to various angles of incidence so as to avoid main rotor downwash at low speeds, and to provide flight stability at high speeds. The positioning means comprises a pair of actuators connected in series, each actuator being independently controlled by separate, uniquely-related electronic control systems, which cause inputs to the actuators so as to jointly accomplish desired incidence angles of the stabilator. The actual position effected by each stabilator is compared with that of the other, along with their rates of change of position. In the event that the position or the rate of positioning of the two actuators fail to track within a threshold magnitude of disparity, an alarm condition is manifested which provides a signal to the pilot and transfers control over the actuators from the electronic control systems to a manual slewing switch. In said application, the positioning of the actuators is compared against fixed limits, such as, for instance, 10.degree. of position and 6.degree. per second of rate of change of position.
It has now been found that the system disclosed in the aforementiond application provides difficulty in establishing the proper limits. For instance, with the limits set at 10.degree. and 6.degree. per second, there are an unacceptable number of nuisance shutdowns; these are shutdowns resulting from variations in inputs rather than in responses of the system. At lower speeds, it isn't necessary that the two systems track as closely as at high speeds, so there is a tendency to suggest opening up the limits to avoid the nuisance shutdowns. However, it is clear that when the limits are opened up to preclude undesirable shutdowns which are not indicative of fault, then they are so great that at high speeds, undesirable maneuvers can be created by faulty stabilator position as a result of failure of the two, completely-separate actuator systems to track properly.
It has also been found that one of the causes of the nuisance shutdowns is a failure of the airspeed sensing system to track properly; that is, that the two separate airspeed systems provide varying inputs to the two actuator control systems, so that even when the control systems are operating properly, they will naturally command different positions due to the different airspeed inputs thereto. And, in fact, the two airspeed sensing systems fail to track by magnitudes sufficient to cause the fault sensing system to operate with the 10.degree. and 6.degree. per second limits thereon.